Incandescent lamp



April 13, 1954 K. D. MCMAHAN INCANDESCENT LAMP 2 Sheets-Sheet 1 Filed Dec. 20 1951 INVENTOR K'eniomDMWak/aiv BY ATTORNEY April 13, 1954 McMAHAN INCANDESCENT LAMP 2 Sheets-Sheet 2 Filed Dec. 20, 1951 EXTENSION OF UFE AT REDUCED VOLTAGE L\ FE PERCENT 600 (AVERAGE LIFE AT RATED VOLTAGE :qfi. AVERAGE LIFE WITHOUT BALLAST @JDO a 'L\FE PERCENT EXTENSION OF LIFE l BY BALLAST 20o INVENTOR Knknfl/WMahan ATTORNEY Patented Apr. 13, 1954 NIT-ED STAT E1 A E OF E.

2,675,592 INCANDESCENT LAMP Kenton .D McMahan, Scotia, N. I Y. Application December 20, 1951, Serial No. 262.539

.7 Claims. 1

The presentinvention relates to means for improving. thelife. and performance of incandescent lamps.

some of the objects of the. present. invention are to provide an improved incandescent lamp which has increased usefullife combined with increased lightv outputeificiency when subjected to either intermittent. or continuous duty, which has the. darkening; on the inside. of its glass bulb .due to vaporization of its filament delayed, thus improving itsusefullif cycle, which lends itself to manufacture on. conventional. machinery for making light bulbs and is thereforeinexpensive to. manufacture, whichcan be connected asan ordinary light bulb in the usuallight socket Without the need of external auxiliary equipment, and which is constructed to save materials which are strategic under periods of national stress, such as Wolfram (tungsten) and copper.

In accordance. with certain features of the. pres ent invention, there is. provided in series with the filament ofanincandescent lamp, an ohmic resistance ballast device serving to reduce initial surges of current of destructive intensity through the filamentwhen the la is first. turned on. I

It has been found that an ohmic. resistance in series withthe filament. acts. notonly to prolong the life of a bulb when. subjected to. frequent on and off cycles, as. forexamplein flash lighting,

also prolongs thelife, of a bulb when subjected to continuous duty. @Therefore, as, a feature of. the.presentinvention,the ohmic resistance device is.designed tocfier electrical resistance in series withthe filament during-the stage when the current isi .first,turned on., to serve as a ballast against, high current surges at this stage and is also designed to offer electrical resistance in series with thefilament after. the current has become stabilized. during continuous. duty.

As a. further featur'efthe ohmic resistance ballast device comprises. aslender. lead connection from'thefilainent to" the'base. of. the stemv tube of thelamon iade' of. high, resistivity material and. having a structural shape with a ratio'of crossrsectional area to. moment, of inertia about its transverse central. axis substantially below I that ofaround solidfwire, thereby imparting greater strengthjand rigidity to said lead connection, while at the same time obtaining therefrom. the required ohmic resistance. to.prol0ng the life ofthe lamp. M0re specifically, this resistance lead connection is, in the form. of; a thin walled tube.

As another feature of the present invention, 7 the ohmic resistan e ballast devi e in; one of its device when the current is flrst turned on. The filler material for that purpose is such as to have high resistance at room temperature and a much lower resistance atoperating temperature.

Various other objects, features and advantages of the present invention are apparent from the following particulandescription and from an inspection of the accompanying drawings, in

[Whichlamps of the improved type shown, in Figs. 1 and 2 with ohmic resistance ballastoperating under continuous and intermittent duty.

Referring to Figs. 1 and2 of the drawings, the ohmic resistance ballastdevice of the present invention is shown applied to a standard incandescent lamp comprising a glass envelope or bulb I'D containing a filament I I, made as for example oftungsten and supported on one end of wires l2, the other ends'being embedded in the upper end of a post l3 of insulating material. The lower end of this post l3 merges into or is connected totheflattened head portion M of a stem tube l5 of insulating material. Lead-in wires I6 and I! in the stem tube 15 are electrically connected at one end to an end contact 18 and to a screw cap or base. 211 respectively. Electrically connected between the other ends of these leadin wires l6 and I! and the ends of the filament H in series with said filament are similar ohmic resistance leaddevices or connections 2! and'22 of the present invention. Each of these leadconnections 2i and 22. comprises a slender element 24.:made. of azhigh resistivity metal andhaving a structural sha e with a ratio of cross-sectional area to moment of inertia about a central transverse axis substantially below that of a round wire and preferably less than 60% that of a round solid wire. More specifically, this lead element 24 is in the form of a thin wall tube having an outside diameter about the same as that of the usual lead wire. For a specific wattage and for certain operating conditions, a tube 24 having a wall thickness of about .0015 inch and a bore of .012 to .015 inch in diameter will give the desired rigidity and desired ohmic resistance. This compares with a solid lead Wire of a standard 50 watt lamp having a diameter of .012 inch.

A lead connection of the structural characteristics described has greater strength and rigidity than a round solid lead of the same metallic cross-sectional area. This greater strength is desirable to withstand the handling of these lead connections in the process of inserting the filament with attached lead connections into the glass bulb during assembling and to withstand shocks and vibrations incident to transportation and use.

The high resistivity tube 24 at its upper end 25 is flattened and bent into the form of a hook for connection directly to one end of the filament At its lower end, the lead-in wire [6 extends snugly into the contracted lower end of the high resistivity tube 24 of the lead connection 2| and is firmly connected thereto, as for example by welding. The other lead-in wire I! is similarly connected into the high resistance tube element of the other lead connection 22.

The filament II is most conductive when cold but as its temperature increases, the resistance also increases. The overall resistances of lead in wires and lead connections in standard bulbs are low, being less than 0.1 ohm. Therefore at the instant the switch is turned on, a sudden surge of current through the filament produces a shock sufficient in many cases to rupture the filament. To prevent this condition, the tube element is constructed of high resistivity metal as described and has a wall thickness sufllcient to give the desired ohmic resistance depending on the lamp wattage and the operating duty of the lamp. Materials having a resistivity at room temperature, 20 C. of above microhms/cm. are considered for the purpose of the present invention to be classified as high resistivity materials, while those below this resistance value are classified as low resistivity materials. To illustrate this classification, the resistivity of several metals and alloys is herein given:

Copper s- Aluminum 2.58 Wolfram (tungsten) 5.50 Nickel 7.80 Iron 10.00 Platinum 10.00 Nichrome (.80 Ni, .20 Cr) 100.00 Radiohm (.785 Fe, .165 Cr, .05 Al) e- 125.00

Therefore, Nichrome or Radiohm can be effectively used for the tube element 24.

The tube element 24 has the necessary ohmic resistance to serve as a ballast against current feature of the present invention, when the lamp reaches operating equilibrium, the high resistivity element 24 is partially shorted automatically to an extent sufficient to reduce materially the overall ohmic resistance of the lead connections 2| and 22, while maintaining a residue of ohmic resistance necessary to extend the life of the lamp during continuous duty. Towards that end, the ohmic resistance lead connections 2| and 22 are of composite construction and each comprises a semi-conductor element or rod 30 extending along the high resistivity element 24 and arranged to short-circuit partially said element, when the operating equilibrium is reached. In the case where the high resistivity element 24 is in the form of a tube, the semi-conductor element 30 is encased within said tube and makes electrical contact therewith along its entire length, thus providing a parallel electrical circuit.

The semi-conductor element 30 is made of a material which exhibits very high electrical resistivity at low temperatures and steeply negative resistivity coefficients with increased temperature. Suitable semi-conductors which have this property and which may be used are designated varistors and thermistors." Varistors consist essentially of silicon carbide sintered in vitreous ceramic. Thermistors, which are preferred, consist of a variety of metallic oxides, sulphides, and mixtures of these materials to obtain difiercnt resistivity characteristics with temperature. Some of the more common of these are nickel oxide NiO, iron oxide FezOa and silver sulphide AgzS. For the lead connection of the present invention, a thermistor for example constructed by sintering silver sulphide AgzS in an inert binder such as titanium dioxide TiOz gives the desired negative resistivity coefiicient at operating temperatures.

As far as certain aspects of the invention are concerned, the inner material for the composite lead connections 2| and 22 may be a carbonic material having infinite or high resistance at room temperature and a lower resistance at operating temperature.

Fig. 4 shows the curves of Life vs. Volts in terms of percentage, derived from a number of conventional lamps tested at both rated and slightly reduced volta e on 60 cycle A. C. power without resistance ballast, under both continuous and intermittent duties, and subjected to a small amount of vibration, such as would be experienced in many domestic and industrial applications. The lamps subjected to intermittent duty were continuously turned on and off every 12 seconds (4 seconds on, 8 seconds off). The average life of the lamp under continuous duty but under operating conditions at rated voltage has been indicated as on this curve and thelife of the lamp under intermittent duty at rated voltage is shown on this same scale to be only a fraction of 1 0%- Fig. 5 shows the curves of Life vs. Volts in terms of percentage derived from a number of conventional lamps tested at rated voltage on 60 cycle A. C. power. These lamps were made by the same manufacturer and were tested under both continuous and intermittent duty under the identical conditions of the aforesaid tests on which the curves of Fig. 4 are based, except that they were tested with varying amounts of ohmic resistance ballast in series with the lamp filaments.

A comparison of the corresponding curves of Flgs.-. 4. and 5 shows: the very large extension of lamp life. bynsingohmic resistance ballast as against the slight extension of life. by a. corresponding reduction in operation filament voltage. Fig. 5 surprisingly shows that ohmic resistance ballastin an A.C. circuit prolongs the life of a lampsubstantially even whensaidlampis sub- ,iected to continuous duty. The ballast necessary to extend the life of a .bulb whichison continuously is less in ohmic. resistance than that necessary toextend similarly the life. of a: bulb which is switched. on and off at frequent intervals'. It has been found inaccordance with the present invention that for maximum lamplife and a minimum .power loss. due to the ballast, where the lamp is turned on andoff on occasions and subjected to small amount of vibration, such as would be experienced in many domestic and individual applications, it is. desirable toprovide an automatically. changeable ohmic resistance ba last having approximately three times the resistance when the lamp is turned off than when it is operating continuously. The automatic changeover is effected by the use of the inner elements 30 of the composite lead connections 2| and 22 with their negative resistivity coeflicient reducing the overall resistance of said connections when operating equilibrium is obtained. The metal tube 24 controls the resistance at room temperature or at the instant the lamp is switched on, because at that temperature, the resistance of the thermistor eement 3%) is of the order of 100,000 times greater than when hot. After the lamp has been operating for a few seconds, the increase in temperature further increases the resistance of the tube 24 but decreases the resistance of the element 3'3 to effect the desired reduction in the composite resistance.

It has been found in accordance with the present invention that compositelead connections 2! and 22 having jointly an ohmic resistance to dissipate wattage within said connections equiva ent. to between six-tenths of one and four percent of the wattage of the filament I I under operating temperature produces the desired results in accordance with the present invention.

The conduction of heat from the ends of the hot filament through the lead wires of conventional lamps reduces lightemission from-the filament and produceshigh thermal stressesat the junction of the filament and the lead wires. Since the resistance lead connections 2| and 22 of the present invention are attached direct y to both ends of the filamentand have ohmic resistance, heat is generated in said connections blockingv the conduction of heat away from the high temperature filament. This desirable effeet is further enhanced by the fact that materia s having high electrical resistance also have low thermal conductivitvwhich further restricts the flow of heat from the filament.

The darkening of the glass on the inside of the bulb, due to the vaporization of the Wolfram filament materially reduces the light output with age. There is therefore a limit to which the life of a lamp may be efficiently extended. Nevertheless, it has been observed in accordance with the present invention that a lamp using ohmic bal ast darkens at a much reduced rate, so that even with its life increased many folds. the ultimate darkening at failure is no greater than that of a conventional lamp not having the extended life.

Iii

The exact theoretical explanation for-thelarge extension of life and better performance of the improved lampof the present invention using ohmic resistance ballast is not positively known. It is recognized that many complicated phenomena influence the structure and properties of materialsat high temperatures. Due to some of these phenomena, the life of incandescent lamps vary widely between lamps of supposedly identical manufacture, and even more widely between lamps of diiferent manufacture. It has been found that extremely small vibration adversely affects average lamp life and widen the difference found among supposedly identical lamps. Observed test results in combination with. known metallurgical theory lead one to believe that lamp filament failures aredue to what is known. metallurgically as high tem-- perature creep, the stresses being produced principally by thermal temperature gradients, often augmented bystresses due to mechanical vibration. It is believed that the ballast: of the present invention materially reduces the magnitude of these thermal stresses for the following reasons:

i 1. The current surges at the instant thelamp is turned on are materiallyreduced resulting in lower thermal stresses at the junction of. the filament H and lead connections 2| and 22 and along the filament due to .rapid thermal expansion.

2. When operatingon alternating. current, the magnitude of the cycic current peaks is reduced when the coolingoi the filament betweencycles is considered, resulting in reduced high frequency thermal stresses. of comparatively low magnitude but of extreme importance because of the freouency.

3. Because of thereduced thermal stresses, the additional stress due to mechanical vibration or external meansisnot as severe.

- 4. The thermal gradients between the filament "l l and the lead connections 2! and 22 and along thefilament are reduced by the special high resistivity and heat generating lead connections.

The following handbook formula is based on operations under'ideal conditions where the lamps are turned on only once and. mounted on a vibrationless structure.

life

It is. evident from the foregoing formula for the operation of lamps that increased. lamp efiiciency expressed as lumens per watt can be obtained by increasing the filament voltage according to the formula indicated. Hence the improvedlamp of the present invention may be operated at much. higher voltage than that required to, counteract thesmall resistance ballast drop andw still maintain. anexpected life far greater than that of conventional lamps without the ohmic resistance ballast. For instance, a conventional lamp rated at volts could be used with an external ballast on a volt supply. For such an increased voltage and average usage, a resistance ballast having a voltage drop of about 1.5 per cent (Fig. 5) would be required to give for continuous duty an average life at rated voltage 01f 800 to 1200 per cent. At the increased voltage, the ballast drop would be 1.8 volts leaving a filament voltage of 118.2 as compared to a rated voltage of 110 for 100 per cent rated lumens per watt.

connections having a substantially uniform cross section substantially along its full length from Using the formula,

13.1 7,1 :(W near said stem tube to said filament and being of such structural form as to have a ratio 011' Then, 5 cross-sectional area. to a moment of inertia about its transverse central axis substantially below that of a solid round wire, to impart self-sustaining rigidity to said lead connections from LUMENS/WAI'I=113.8%

which when corrected for power loss in the ballast gives, near said stem tube up to said filament.

LUMENS/WATT=112% l0 2. An incandescent lamp as described in claim and 1, wherein each of said lead connections is of NF 17 such structural form as to have a ratio of crosssectional area to moment of inertia about its The above formula for lamp life as a function transverse central axis not substantially more of operating voltage does not truly represent the than 60% that of a solid round wire. relationship for lamps with ohmic resistance 3. An incandescent lamp as described in claim under actual conditions, since under actual aver- 1, wherein each of said lead connections is in age conditions (lamp turned on occasionally and the form of a tube. subjected to small amount of vibration), the 4. An incandescent lamp as described in claim actual life of conventional lamps is only a small 20 1, wherein said lead connections together have fraction of their rated life. It has been found sufiicient ohmic resistance to dissipate between in accordance with the present invention that six-tenths of one and four per cent of the watthe operating conditions make little or no difiertage of said filament at operating temperatures. ence in the life of the lamps with ohmic resist- 5. An incandescent lamp as described in claim ance ballast and that the improved lamp under 1, wherein each of said lead connections is in any condition is extended several fold in life. the form of a tube, and wherein said lead con- Figs. 1 and 2,. show lead connections 2| and 22 nections together have sufficient ohmic resistance of the composite form, each combining both the to dissipate between six-tenths of one and four tube element 24 of high resistivity metal and the per cent of the wattage of said filament at operinner element of material having a negative 30 ating te p sresistivity coefiicient. However, as far as certain 6. An incandescent lamp as described in claim aspects of the invention are concerned, the inner wherein each of said lead connections defines element 30 may be eliminated and the lead cona hcllOW te d therealcng, d ere t e nection may be in the simple form shown in Fig. lamp in l n element f n v istivi y 3 and consist merely in a lead element 24a of cocfiicient in each of said hollows in parallel with structural shape with a ratio of metallic crosst e po d g lead connection. sectional area to moment of inertia about its An incandescent p as described in claim transverse central axis substantially below that whe ea h of Said lead connections is in of a solid round wire. This element 24a for the form of a tube, and wherein the lamp the lamp is preferably in the form of a t b 40 cludes an element of negative resistivity coefficient connected at; its per end t n nd of th in the hollow of each lead connection in parallel filament ll through a hook iformation 25a and W the corresponding d on e tionconnected at its lower end to a lead-in wire l6" and is made of metal having an ohmic resistance and preferably high resistivity sufiicient to dissipate with the other lead connection of said lamp between six-tenths of one and four per References Cited in the file of this patent UNITED STATES PATENTS cent of the Wattage of the filament at operating l-fi l l z lggg temperatures; 1,365,481 Gilson Jan. 11, 1921 Wl'llle the invention has been descr bed Wlth 1 721 292 Britsch July 16 1929 particular reference to specific embodiments, 1t 1,331,052 Weichelt 10, 1931 is to be understoodv that it is not to be limited 0 9 77 Braselton May 5 9 thereto but is to be construed broadly and re- 21,39,773 Braselton May 5 1935 stricted solely by the scope of the appended 2,081,301 Dunkel May 25,1937

e 2,214,742 Laise Sept. 17, 1940 What claimed 15: 2,457,734 Sargent Dec. 28, 1948 1. An incandescent lamp comprising, an en- 2 433 247 st t 27 1949 relope, a stem tube, a filament, and lead con- 2,646,439 McArron "31113721, 5

nections to the ends respectively of said filament in series with said filament, said lead connections FOREIGN PATENTS being of metallic material and having a resistivity Number Country Date at 20 C. of above 1 0 1nicrohms/cm. each ofsaid 423,078 Great Britain Jan. 24, 1935 

